The invention relates to a process of making cement clinker and to an apparatus for performing said process.
The process of making cement clinker may be subdivided into three different steps. In the first step, the finely ground raw material is heated to about 600.degree. C. In this step, no essential reactions take place, with the exception of the dehydration of the clay (alumina) minerals. The second process step is characterized by the highly endothermic reaction of deacidifying the calcium carbonate. The third process step comprises further heating of the material to sintering temperature and the mildly exothermic reaction of clinker mineral formation. From the viewpoint of energy, the deacidification of the calcium carbonate is most important, as according to H. Kuhl "Zementchemie", VEB-Verlag Technik Berlin 1959, page 354 (Volume II), it entails an evolution of heat of 1638 kJ/kg. Since conventional cement raw meal contains about 75 percent of limestone, the main share of energy in the cement burning process is required in the deacidifying step, i.e. within a temperature range of about 650.degree. to 950.degree. C.
Taking this fact into consideration, early attempts were made to separate the individual steps of the cement production process. The most widely known processes developed on this basis are the Lepol process and, above all, the heat exchanger process in which preheating and part of the deacidification take place on a grate or in cyclone heat exchangers. In both processes, the transition of heat between the hot combustion gases and the raw material is greatly improved as compared to the long rotary kilns customary during the initial era of cement production. In both processes, however, a relatively low degree of deacidification is achieved on the grate or in the cyclones. So, for instance, only about 20 to 25 percent of the limestone are deacidified in a conventional heat exchanger furnace after the last cyclone stage or in the furnace inlet zone. This degree of deacidification which is relatively low in view of the temperatures within the furnace inlet zone is explained, a.o., by the fact that the sojourn time of the raw material to be deacidified in the heat exchangers is comparatively very short.
In the course of rationalization measures, socalled secondary burners have been provided in the zone of the last cyclone stage during the past ten years, so as to concentrate the energy supply on the deacidification zone. This greatly relieves the sintering zone of the furnace, although the combustion air required for the fuel introduced by the second burner must usually be supplied through the furnace. For this reason, a further development provides for the combustion air required for the secondary burners to be no longer supplied as secondary air, but instead separately as tertiary air, preferably as preheated hot cooler air. In this way, the furnace can be further relieved.
In the practice of cement clinker production, two variants of the use of secondary fuel have been applied to a fairly great extent.
In the first variant, secondary fuels are introduced into heat exchanger furnaces of conventional construction in the temperature range of about 650.degree. to 1000.degree. C., i.e. as a rule in the range between furnace inlet and lowest cyclone stage of the heat exchanger. This can normally be achieved without high investments. The secondary fuels used are heavy fuel oil, gas, coaldust, but also waste products of constant calorific value such as scrap tires. The gaseous, liquid or fine-grained fuels are burned in admixture with the raw meal coming from the heat exhanger; the coarse-grained materials, such as scrap tires, are burned at the furnace inlet or conveyed into the furnace where they are burned. The amount of secondary fuel to be used is limited in both cases. It is normally not possible to meet more than 20 to 25 percent of the total energy requirement in this way by using secondary fuels. If this value is exceeded, the Co-content in the furnace exhaust gas is greatly increased, the exhaust gas temperature rises together with the total energy requirement. The degree of calcination cannot be essentially increased in this way by the use of secondary fuels. It is therefore necessary to supply the amount of energy required in the furnace for complete deacidification via the primary burner of the furnace. According to past experience, this is no longer possible if the share of secondary fuels is too high.
The use of coarse-grained materials such as scrap tires is on principle successful when they are introduced into the kiln itself where an adequate degassing and burning time is available. It is naturally not possible in this case to use tertiary air, instead, the entire air required for the combustion of these secondary fuels must be supplied through the kiln. This results in an essentially increased rate of gas flow and thus an increased dust circulation, and, as a further consequence, in the transfer of already deacidified hot raw meal from the kiln into the heat exchanger. The heat is, so to speak, abducted from the kiln into the heat exchanger. This results in increased scab (scar) formation in the heat exchanger and a corresponding increase in the total energy requirement.
The second variant, which is pratically used in new plants only, provides for the combustion of the secondary fuels in a separate combustion chamber which is frequently called calcinator. Several suggestions for such calcinators have been made. They all have in common a prolonged sojourn time of the raw meal, high investment rates and an increased input of electric energy due to the pressure drop in the calcinator. Only high grade fuels are suitable as secondary fuels. In order to relieve the sintering zone, the air required for the combustion of these fuels is frequently supplied to the calcinator not through the kiln, but via a separate line, preferably as hot cooler air. Suitable calcinators can achieve a degree of calcination of more than 80 and up to about 95 percent. Difficulties frequently arise because this operation results not only in the deacidification of the raw meal, but also in the formation of melt phases and scab which change the flow rate of the gas and impede the transport of the deacidified raw meal into the kiln.